A vehicle needs to detect the rotational speed of vehicular wheels in order to control an antilock brake system (ABS) and a traction control system (TCS). It is therefore a common practice to mount a sensor portion to the cover fitted in one end of an outer ring member of a vehicular-wheel bearing assembly for rotatably carrying the vehicular wheel on a suspension of the vehicle. The sensor portion detects an inner ring member of the bearing assembly or a detected portion fixed to the inner ring member, whereby the rotational speed of the vehicular wheel is detected.
Conventionally, the sensor portion is constructed independently from the cover and is fixed to the cover as inserted in a through-hole formed in the cover. More recently, however, it has been proposed to unify the sensor portion with the cover by the molding of synthetic resin, as shown in FIG. 2, for the purposes of increasing the positioning precisions of the sensor portion and reducing assembly costs (see, for example, Japanese Unexamined Patent Publication No. 2001-194376).
In FIG. 2, a cover 31 formed of a metal sheet is fitted in an inside surface of one end of an outer ring member 32. A sensor portion 37 including a connector 35 for supporting a terminal 34, and a sensor element 36 is molded of the synthetic resin and onto a through-hole 33 formed in the cover 31, so as to be unified with the cover 31.
However, a sensor mounting structure disclosed in the above patent publication has the following problem. A synthetic-resin portion 38 located inwardly of the cover 31 is shaped like a thick block which has a relatively small surface area for its volume. A radial dimension (with respect to a vertical direction as seen in FIG. 2) of the resin portion 38 is smaller than that of a flange 35a of the connector 35 in contact with an outside surface 31a of the cover 31, while an axial dimension (with respect to a transverse direction as seen in FIG. 2) of the resin portion 38 is substantially equal to the radial dimension thereof. Therefore, the resin molding process takes a substantial length of cooling time so that the resin portion tends to suffer the formation of cavities therein, failing to attain a sufficient strength. This leads to the problem that the sensor portion 37 may suffer breakage at its portion fixed (mounted) to the cover 31 so as to disengage from the cover 31.
The synthetic-resin portion 38 having the sensor element 36 embedded therein has the radial dimension (with respect to the vertical direction as seen in FIG. 2) substantially equal to that of the connector 35 located on the outside surface 31a of the cover 31. To provide a capacitor for preventing the noise invasion into the sensor element 36, the synthetic-resin portion 38 is increased in the axial dimension (with respect to the traverse direction as seen in FIG. 2) so that the whole body of the bearing assembly is increased in size accordingly. This leads to another problem that the bearing assembly is decreased in the degree of design freedom.
Furthermore, the connector 35 and the synthetic-resin portion covering the sensor element 36 are required of different degrees of strength. In a case where these components are formed of the same material, the more stringent one of the specifications (one for the synthetic-resin portion of the connector 35) must be adopted. Hence, the bearing assembly as a whole has an excessive quality, which results in cost increase.
In addition, the integral molding process requires large molds, which have complicated configurations. It is therefore far from easy to produce the molds. This constitutes another factor of the cost increase.